1. Field of the Invention
An object of the present invention is a device for the melting of a fuse in a CMOS type integrated circuit. It can be applied in many, varied integrated circuits, in particular, to define read-only memories or to replace functional circuits therein, which have been put out of order, by redundant functional circuits. In memory card applications, fuses can also be used for the protection, in access, of certain memory zones. Before they melt, the connection that they set up enables the integrated circuit of the memory card to be programmed in order to introduce secret codes or recognition algorithms therein. After they melt, this programming as well as the reading of these secret codes or algorithms become impossible.
2. Description of the Prior Art
In an integrated circuit, fuses are made chiefly by means of polysilicon or metal connections. The dimensions of the cross-section of these connections as well as the useful melting length of the fuse are defined as a function of a nominal melting current. The principle of the melting of a fuse is simple. It is enough to make a sufficiently high current flow through the fuse to make it melt. In practice, in modern integrated circuits, this current is in the range of a few tens of milliamperes. After the fuse has melted, one of the terminals of the fuse, namely the one to which it is sought to make access impossible, is electrically unconnected. To prevent this kind of situation, this terminal of the fuse is connected to a level detector. This level detector is capable, firstly, of revealing the melting of the fuse and, secondly, of keeping the end of the unconnected connection at a constant potential.
However, there are problems related to the melting of the fuse. First of all, the melting current should be fairly high. This means that there should be a fairly large control transistor to convey this current. This control transistor can be used to facilitate the programming of the melting of several fuses of a circuit. The size of the transistor is related to the current which this transistor should let through. Furthermore, the rising edge of the current pulse should be extremely steep. For, if the rise in current is slow, make every due allowance, the fuse melts slowly. As and when it melts, its electrical resistance increases. For example, the dimensions of the section may be reduced during this melting process. Since this resistance increases, the energy that the fuse can dissipate is reduced. Since this energy is reduced, the fuse gets heated less and less, with the risk that it can no longer melt. If this phenomenon occurs, it then becomes impossible to make the fuse melt. Finally, the command of the transistor which conveys the current makes it necessary to provide for the application, in the melting procedure of a fuse, of a command to end the melting process, and this complicates this procedure. In practice, a melting period has to be planned and this melting period should be relatively big to provide for efficient melting under all circumstances.
Moreover, fuses are highly fragile with respect to electrostatic discharges. This fragility is essentially due to the presence of the big control transistor for the melting of the fuse. For, owing to its size, this transistor no longer forms an efficient shield against high voltage electrostatic discharges, even if its control input is deactivated. For, it is possible that this big transistor lets through the energy of the electrostatic discharges when the integrated circuit is handled, especially at the end of manufacture and when this integrated circuit is being installed in an electronic system. These repeated electrostatic discharges may have the same effect as an excessively slow rising edge of the melting current. At the instant when it is desired to make the fuse melt, its resistance may be great because of the initial melting that it may have undergone. The energy that it can dissipate is then excessively low: it no longer melts.
An object of the present invention is to overcome these drawbacks while, however, being, at the same time, applicable only to CMOS type integrated circuits. This restriction is not troublesome because CMOS technology is presently coming into ever-increasing use. In particular, it is often preferred to NMOS technology. CMOS technology is characterized by a semiconducting substrate of a given type (generally P type) of conductivity in which insulation pads are made by implantation of impurities corresponding to an opposite type of conductivity (generally of the N type). Various transistors and junctions can be made by implanting doped regions with one type of impurity or another either in the substrate or in these pads.
However, CMOS technology is known to suffer a conduction "flip-over" or "latch-up" phenomenon. This latch-up phenomenon corresponds to a triggering of stray thyristors which are naturally created in a CMOS circuit with pads. These stray thyristors are formed by a sequence of regions of alternate types of conductivity. A first region of a given type of conductivity (generally P) is contained in a pad of a second type of given conductivity (generally N). This pad is itself placed in a substrate of the first type of given conductivity (generally P) and has one or more regions of the second type of conductivity (generally N). The succession of these four regions of alternating conductivity forms a thyristor. In certain cases, keeping this thyristor in the off state is a difficult task.
In the invention, however, the existence of this stray thyristor is turned to advantage in order to make the fuse melt by connecting a terminal of this fuse to this thyristor and by triggering it. At the instant of melting, the current is given by a melting potential generator. The current flows through the fuse and the thyristor, which has one of its terminals connected to the other pole of the melting potential generator. By acting in this way, an immediate solution is got to the problems referred to. Firstly, the thyristor, while being smaller, accepts a higher current. Secondly, the thyristor is conductive as long as the fuse is not completely melted: namely as long as the fuse lets current through. As soon as the fuse has melted, the thyristor gets deactivated. There is no need to provide for a command to end the melting process in this procedure. Furthermore, it will be shown that when this stray thyristor is not triggered, it has a shield for the fuse, against the electrostatic discharges, which is far greater than in the prior art referred to.
An object of the invention is therefore a device for the melting of a fuse in a CMOS type integrated circuit comprising a thyristor, series mounted with the fuse, said thyristor being a stray thyristor of this integrated circuit, and means to control the turning on of this thyristor.